The Case of the Bendable Blade

Band saws are useful metal cutting tools. Cold-worked steel blades are adequate for cutting mild steel and most non-ferrous alloys. Such steels dull quickly on cutting hardened steels and are also easily softened by a temperature rise. Hacksaw blades are subject to the same limitations.

So-called hot working tool steels may be heat treated to high hardness and in addition are not softened by most thermal excursions. Unfortunately, flexing the hardened tool steel during operation would give very early fatigue failure. The teeth clearly need a much greater hardness than is tolerable in the rest of the blade.

A flexible carbon steel blade with hard tool steel teeth seemed ideal. But how to join the two? An arc welding process would melt the thin strip needed for the teeth. Then about 40 years ago electron beam welding came into use. The resulting welds have a very high depth-to-width ratio and may be used to join very thin strips to the edge of larger strips.

Some 40 years ago a major tool manufacturer, here referred to as D (for defendant), was using this process to weld a thin strip of tool steel to a carbon steel backup. The composite blade was then heat treated to produce band saw blades with very hard teeth and a flexible, fatigue-resistant backup. The heat treatment was that for the tool steel, which should have given wretched properties in the backup. Instead the backup was fine. Overheating prior to quenching seemed to cancel out the effects of overheating during tempering. This counterintuitive discovery is the stuff of patents. The bimetallic blades became a profitable product line.

Then, like a bolt out of the blue, another major tool manufacturer, referred to as P (for plaintiff) sued D for patent infringement. It seems P had a patent on the process for making bimetallic band saw blades. D pish-tished the patent as they had been making bimetallic hacksaw blades for a long time, constituting (in D's opinion) prior art. The case went to trial and to the horror of D and their law firm, they lost! P and their experts convinced the judge that hacksaw and band saw blades were sufficiently different in usage that P's patent was valid. D's law firm was so devastated that it offered to take the appeal on contingency and receive nothing if the appeal failed and only their usual fee if successful.

The appeal was based on a 1950's German patent for a bimetallic band saw blade comprised of a strip of tool steel electron beam welded to a carbon steel backup. The patent predated all the others and was cited by D as prior art. The plaintiff demurred, noting that the German backup steel had a different composition than the steel in their patent. The Federal Appeals Judge upheld the appeal and sent the case back to the original court with instructions to find whether or not "one skilled in the art" upon reading the German patent would be likely to use P's alloy. At his point my colleague, Prof. Regis Pelloux, and I were called in as experts to help prepare for the partial retrial and to appear as witnesses.

The German steel used more manganese and silicon in lieu of the chromium used in P's steel. (I suspect that chromium was in short supply in post-WWII Germany.) A little digging in the literature showed that the two backup steels responded virtually identically to heat treatment. Such equivalence is not unusual in steels as alloying elements may often be interchanged to give the same mechanical properties.

Pelloux and I testified to the equivalence of the two steels, while P's expert demurred most vigorously. The trial judge reversed himself, which judges hate to do, and found for D, declaring P's patent invalid. Joy ran unconfined in D's camp. My fee for time spent on this case was a major part of the down payment on a home for my family.

Today there are many different kinds of band saw blades, each with a specific set of uses. I believe the bimetallic blade was the first step beyond the cold-worked material.

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